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          <h1 class="post-title" itemprop="name headline">《我要进大厂》系列之ReentrantLock与synchronized两种锁的区别</h1>
        

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        <div id="vip-container"><p>在大厂面试中，特别是第一轮技术面试中，锁是一个非常高频的主题，在JDK中主要提供了两种方式：JUC(JAVA并发框架)中的ReentrantLock与synchronized关键字。</p>
<p>那这两种有何区别呢?在实际工作中又如何选取呢？</p>
<p>ReentrantLock 实现 synchronized 的语义，提供多线程的<strong>同步阻塞语义</strong>，即我们通常意义上的<strong>同步锁</strong>，但 ReentrantLock 的使用更加灵活，提供了synchronized关键字不具备的功能特性。</p>
<h2 id="1、可重入性"><a href="#1、可重入性" class="headerlink" title="1、可重入性"></a>1、可重入性</h2><p>ReentrantLock 与 synchronized 都是允许重入的。</p>
<p>用一段的代码说明下锁的可重入语义：<br><img src="https://img-blog.csdnimg.cn/20210327211011459.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>即一个线程在获取了一把锁后，还可以继续申请获取。</p>
<h2 id="2、ReentrantLock支持公平-非公平锁"><a href="#2、ReentrantLock支持公平-非公平锁" class="headerlink" title="2、ReentrantLock支持公平/非公平锁"></a>2、ReentrantLock支持公平/非公平锁</h2><p><img src="https://img-blog.csdnimg.cn/20210327211439929.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>NonFairSync 非公平锁：在申请锁时首先尝试去<strong>抢占锁</strong>，如果未成功获取锁后才无奈进入排队队列。</p>
<p>FairSync 公平锁：在申请锁时会进入锁排队队列，中规中矩的等待锁。</p>
<p>例如当前有3个线程在排队等待锁，持有锁的线程执行完临界区代码后释放锁时，正好又来了一个新的线程也需要获取锁，<strong>如果是公平锁，则需要等待前面3个线程获取锁后才能获取锁</strong>。如果是<strong>非公平锁</strong>，他会尝试与锁排队队列总的其他线程一起竞争锁，<strong>有可能直接获取锁就执行，这对已在队列中排队的线程就显得不那么公平了。</strong></p>
<p><strong>通常情况下，非公平锁涉及更少的线程切换开销，整体性能比公平锁要高</strong>。</p>
<h2 id="3、ReentrantLock支持锁等待超时"><a href="#3、ReentrantLock支持锁等待超时" class="headerlink" title="3、ReentrantLock支持锁等待超时"></a>3、ReentrantLock支持锁等待超时</h2><p>ReentrantLock 在申请锁时支持设置超时时间，即在多久未成功获得锁时被自动唤醒，可通过 tryLock 方法实现。<br><img src="https://img-blog.csdnimg.cn/20210327213033602.png" alt="在这里插入图片描述"><br>该方法获取锁后会立即返回，否则会阻塞，直到超时，返回true时表示成功获取锁。</p>
<h2 id="４、ReentrantLock-支持中断"><a href="#４、ReentrantLock-支持中断" class="headerlink" title="４、ReentrantLock 支持中断"></a>４、ReentrantLock 支持中断</h2><p>ReentrantLock 提供了 lockInterruptibly()  方法实现<strong>可中断的锁申请</strong>，即一个线程在竞争锁而陷入阻塞状态后，可以由<strong>另个一个线程</strong>来中断这个线程，从而放弃锁申请。</p>
<p>其实现是通过LockSupport的park()方法。<br><img src="https://img-blog.csdnimg.cn/20210327214416950.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>LockSupport 的 park 方法能响应中断，如果线程被中断，<strong>该方法能停止阻塞而被返回，但不会抛出 InterruptedException</strong> 。</p>
<h2 id="５、ReentrantLock-支持灵活的监控"><a href="#５、ReentrantLock-支持灵活的监控" class="headerlink" title="５、ReentrantLock 支持灵活的监控"></a>５、ReentrantLock 支持灵活的监控</h2><p><img src="https://img-blog.csdnimg.cn/20210327231454357.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>ReentrantLock提供了众多的方法，对开发监控锁相关的应用程序有莫大的帮助。</p>
<h2 id="６、总结"><a href="#６、总结" class="headerlink" title="６、总结"></a>６、总结</h2><p>ReentrantLock与synchronized具有相同的同步阻塞语义，从jdk1.6以后，两者的性能相差无几，如果没有特殊的使用需求，直接使用synchronized即可，因为锁的内部存储结构由虚拟机负责，使用简单。</p>
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